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Chemistry Letters Vol.35, No.5 (2006)
Synthesis and Electrophosphorescent Properties of a Novel Iridium Complex
with Arylamine Unit in the Polymer Light-emitting Devices
Zhonglian Wu,1 Kongqiang Xing,3 Cuiping Luo,1 Yu Liu,1;3 Yuping Yang,1 Quan Gan,1 Meixiang Zhu,1
Changyun Jiang,2 Yong Cao,2 and Weiguo Zhuꢀ1
1College of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China
2IPOMD, South China University of Technology, Guangzhou 510640, P. R. China
3Department of Chemistry, Qiongzhou University, Wu Zhishan 572200, P. R. China
(Received January 6, 2006; CL-060014; E-mail: zwg18@xtu.edu.cn)
A novel Ir complex, (BuPhNPPy)2Ir(acac) with arylamine
Ir complex as dopant with 4% weight concentration and a blend
of poly(vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphen-
yl-1,3,4-oxadiazole (PBD) as host matrix, a single-layered
PLED exhibits a high EQE of 10.4% and bright green emission
at 533 nm. To the best of our knowledge, (BuPhNPPy)2Ir(acac)
is the first example of triarylamine-containing Ir complex with
highly luminous efficiency.
The synthetic route of this (BuPhNPPy)2Ir(acac) is shown in
Scheme 1. BuPhNPPy ligand was prepared by the modified
Ullmann reaction between 4-(2-pyridyl)phenylamine and 4-
tert-butyliodobenzene in refluxing o-dichlorobenzene.13 The
chlorobridged dimer and the bicyclometalated (BuPhNPPy)2Ir-
(acac) complex were synthesized according to a improved
method.5 The typical synthetic experiment of this Ir complex
was completed below.
Iridium trichloride hydrate (0.179 g, 0.0508 mmol) and
BuPhNPPy (0.881 g, 2.03 mmol) in a mixture of 2-ethoxyethanol
(15 mL) and water (5 mL) were refluxed for 25 h under nitrogen
atmosphere. The yellow solid was isolated by filteration and
washed with 10 mL of ethanol and 5 mL of acetone, respectively,
to give chlorobridged dimer of Ir2(BuPhNPPy)4Cl2 (2) after a
majority of 2-ethoxyethanol was removed out under vacuum.
Dimer 2 was crystallized from the mixed solvent of dichloro-
methane, toluene and hexane (1:1:1) to give 180 mg of yellow
crystal of dimer 2. Whereafter, dimer 2 (150 mg, 0.0685 mmol),
acetyl acetone (180 mg, 0.018 mmol) and sodium carbonate
(79 mg) in 2-ethoxyethanol (15 mL) were refluxed for 15 h under
nitrogen atmospheres. Excess water was added in the residue af-
ter a majority of 2-ethoxyethanol was evaporated out under vac-
uum, and resulting residue was cooled to room temperature. The
yielding yellow precipitate was filtered, dried, and chromato-
graphed using aluminum oxide/dichloromethane column to give
100 mg of (BuPhNPPy)2Ir(acac) powder with a yield of 60.0%.
unit, was synthesized. A peak external quantum efficiency of
10.4% photons per electron at current density of 32 mA/cm2
and a maximum luminance of 14610 cd/m2 at drive voltage of
24 V were achieved in a single-layered polymer light-emitting
device using this Ir complex as dopant with 4% dopant concen-
tration and a blend of poly(vinylcarbazole) as host matrix. The
highly efficient electrophosphorescence of the Ir complex was
exhibited with green emission at 533 nm.
Cyclometalated iridium (Ir) complexes have been of consid-
erable interest in electrophosphorescence (EPH) because they
have strong spin–orbit coupling of the 5d orbit resulting in effi-
cient intersystem crossing from the singlet excited state to the
triplet excited state.1–9 Most of these emissive cyclometalated
Ir complexes with imine ligand were homoleptic complexes1–4
and heteroleptic complexes5–9 in the early studies. The strong li-
gand field of the imine ligand, along with the added stabilization
of ꢀ donation from iridium into the aromatic ligand, helps to
make these types of complexes very stable and exhibit signifi-
cant optoelectronics properties in the phosphorescent polymeric
and organic light-emitting devices (PLEDs and OLEDs).10 High-
ly efficient phosphorescent OLEDs and PLEDs with Ir com-
plexes as dopants and organic small molecules or polymers as
hosts matrix were recently achieved. An external quantum effi-
ciency (EQE) of 19% photons per electron (ph/el) in OLEDs
and 10% ph/el in PLEDs using Ir complexes as green phospho-
rescent emitters were achieved by Ikai11 and Gong,12 respective-
ly. Despite these developments, attempts to further expand the
potential of Ir complexes for practical application in OLEDs
and PLEDs have encountered some obstacles. For example,
the high efficiency of the devices was obtained at low current
density. EQE decreases with increased current density (J) in
the devices after J passed a critical level. In addition, the molec-
ular aggregation tendency of Ir complexes was enhanced in the
higher J, which leads poor device efficiency. To realize the prac-
tical applicaton of Ir complexes in highly efficient PLEDs, it is
critical to design the rational molecular structure of Ir complexes
to effectively suppress the molecular aggregation effect.
In this paper, N,N-di(4-tert-butylphenyl)-4-(2-pyridyl)ben-
zenamine (BuPhNPPy) is designed and chosen as the cyclome-
talating ligand with arylamine unit to prepare a novel bicyclome-
talated Ir complex. As a arylamine unit with bulky butyl group
was incorporated into the Ir complex, the reduction of aggregat-
ing and self-quenching tendency of Ir complex should drastically
occur, which results in highly luminous efficiency and the
improved optoelectronic properties for Ir complex. Using this
Scheme 1. Synthetic route of the cyclometalated Ir complex.
Copyright Ó 2006 The Chemical Society of Japan
Wu, Zhonglian
